Materials and Methods

Animals. The Roman L. Hruska U.S. Meat Animal Research Center ( MARC) Animal Care and Use Committee approved the use of animals in this study. Crossbred steers and heifers (n = 66) of 25, 50, or 75% Piedmontese inheritance were produced. Matings were such that approximately 25% of animals were ex- pected to be homozygous normal for the “double muscling” gene, 50% of animals were expected to be heterozygous for the double muscling gene, and 25% of animals were expected to be homozygous for the double muscling gene (i.e., 0, 1, or 2 copies of the gene for double muscling). Animals were weaned at ap- proximately 200 d of age and were given ad libitum access to corn-corn silage diets from weaning to slaughter at 12 to 15 mo of age. Following weaning, the energy concentration of the diet was steadily increased over a 90-d period until the steers reached the finishing diet (3.14 Mcal of ME/kg of dry matter). Animals were serially slaughtered to further increase the level of variation in carcass composition and marbling scores.

Carcass Grade Data. All cattle were slaughtered at the MARC abattoir and carcasses were chilled for 48 h (24 h at 1C; 24 h at 1C). The right carcass side was ribbed conventionally between the 12th and 13th ribs, USDA quality and yield grade data were recorded, and lean color (1 = very light cherry-red, 2 = cherry-red, 3 = slightly dark, 4 = moderately dark, 5 = dark red, 6 = very dark red, 7 = black) was scored. An outline of the perimeter of the longissimus was traced onto acetate paper and longissimus area was deter- mined using a Microcomp PM morphometer (Southern Micro Instruments, Atlanta, GA). In subse-

65 cm from the center of the Images were captured using camera (DXC-970MD/1, Sony equipped with a 25-mm f2.8 

Video capture translated a 24  32 cm area at the steak surface into a 480  640 pixel array for each RGB channel. Specifically, the camera was positioned such that the tip of the camera lens was 122 cm from the surface of the steak and the f-stop was set to 11. Images were captured and analyzed auto- matically using a macro computing program (http:// shack.marc.usda.gov/MRU_WWW/protocol/image_ analysis_macros.pdf), which was recorded and edited by the authors. The variables measured by image analysis (Figure 1) included total lean area ( LEAN), total fat area ( FAT), and total steak area ( TOTAL). Histogram ranges used to identify each component are

described identified PIECE) ,

in Table 1. The largest lean portion was

by autoclassification, and red intensity ( RED),

its area ( EYE- green intensity

( GREEN) ,

blue

intensity

( BLUE) ,

and

density

quent discussion, that measurement carcass longissimus area ( CLA).

is

referred

to

as

( DENSITY) (LEAN, FAT,

were determined. All components TOTAL, and EYEPIECE) were meas-

Image Analysis. A 2.54-cm thick steak was removed from the left carcass side using a double-bladed reciprocating saw as described by Shackelford et al. (1997a). Two straight, parallel cuts (2.54 cm apart) were made simultaneously through the posterior half of the 12th thoracic vertebra, longissimus, and adja- cent fat perpendicular to both the long axis and split surface of the vertebral column. The cut proceeded to a point approximately 15 cm lateral to the lateral tip of the longissimus. A cut was then made perpendicular to the first two cuts to separate the lateral end of the steak from the carcass at a point 8 cm from the lateral tip of the longissimus. The cut surface of each steak, which was often smeared with fat during the cutting process, was scraped to improve fat/lean contrast.

( HOLEAREA) ,

and

percentage

hole

area

( PERHOLE) was calculated. Percentage lean ( PER- LEAN) was calculated for each steak as 100  LEAN/ (LEAN + FAT) rather than 100  LEAN/TOTAL because preliminary analysis indicated that the former was more highly related to carcass cutability than the latter.

For each image analysis steak, an outline of the perimeter of the longissimus was traced onto acetate paper and longissimus area was determined using a Microcomp PM morphometer (Southern Micro Instru- ments). In subsequent discussion, that measurement is referred to as steak longissimus area ( SLA).

Carcass Yield. The right side of each carcass was fabricated into boneless, totally trimmed retail product according to Wheeler et al. (1997). Each